Rebel Rye Porter, SG too low- seeking corrective suggestions

So this is my second AG batch. The first was NB's Smashing Pumpkin Ale which was a great success. This time around I adjusted my process a bit to get closer to the strike temps (pre-heated the mash tun).
For my second AG, I went for the Rebel Rye. I hit the mash temp of 151 dead on. I missed the mashout temp (target: 170, actual: 162) since my weak kitchen stove didn't get the water to boiling in time. But I continued on, re-circulated, and started fly sparging. I collected ~7gal of wort and everything seemed great until I measured my pre-boil SG (1.038, temperature corrected). Using Brewer's Friend OG/FG calculator (http://www.brewersfriend.com/allgrain-ogfg/), I got an estimated pre-boil gravity of 1.044- way off!
Fortunately I had had DME on hand and added some to bring up the SG, but I'm bummed that after hitting all the temperatures (minus the strike-temp), the pre-boil SG was too low! Probably even more so since I was dead on with the pre-boil gravity for the 1st batch. Since I'm new at this, I'm really want to know if there's something I can do to correct this. Below is the grain bill

7lbs Rahr 2-row
2lbs Weyermann Rye Malt
2lbs German Dark Munich Malt
1lbs English Medium Crystal
0.5lbs English Black Malt
(Grain was crushed by Northern Brewer)

Sacch rest: 151F
Mashout: 170F

The only difference in my process between the 1st batch and this one was that I collected 7gal of wort instead of 6.5gal and added about a gallon more water at mashout in an effort to hit the mashout temp (missed it on my first batch too, but probably because I didn't add enough water). Any suggestions would be greatly appreciated.

I've found correction calculators will get you sorta close if your within 15-20 deg of your calibration temp. Beyond that and IMO they are crap. Next time try to cool your sample down significantly before taking a reading. Also, its not the end of the world to continue your mash an extra 15min while your getting your mash out or sparge water up to temp.

Thanks, I didn't believe my first reading so I let a sample sit for an hour and it was still 1.038 at room temperature. Any thoughts on what may have caused my low efficiency even though the temps were correct?

501irishred, thanks for tip on letting the mash sit while the water heats- it actually got me thinking about what I might have done wrong!
After going back over things, I realized that I mashed out 10 minutes earlier than I should have (at 50 minutes through the Sacch rest).
So overall, I only had a 50 minute Sacch rest and a total mash time of 60 minutes instead of 70 minutes.
I'm not sure, but this may account for at least part of my low efficiency problem and I would have been at least closer to my target pre-boil OG.
could 10 minutes in the Sacch rest make that big of a difference in terms of hitting the OG?

I would recommend doing a search on this site for "mash efficiency". There are a ton of things that could affect you here like strike temp, sparge temp, water PH, grain crush size, sparge/runoff time, mash tun size...........and the list goes on.
Before you begin changing a lot of things to correct efficiency, try another batch. Try to pay special attention to strike temps. If you're confident about the things you did try, it will make troubleshooting much easier. Who knows, it may come out perfect, and you can chock this batch up to an anomaly.

Funny, I was in the same boat. The Rebel Rye was my second AG batch, the first coming out perfect, hitting my target gravities without problem. I felt like my Rebel Rye mash went perfectly but ended up getting an amazingly low OG as well.
Luckily I had some amber dme around, but only enough to get an OG of 1.056. I really thought about all of my process and can only think that I either did not stir enough (not really sold on that one) or there was a less-than-great crush. Either way, its now been sitting in secondary on the oak and rye whiskey infusion for almost two weeks and tasted promising during the transfer. Hope yours turns out well!

Yeah, I threw in 1lb of light DME and when I tasted it during the transfer it seemed pretty good but with a more subdued malt profile than I would have hoped for. But the rye was still there so we'll see how it is after a month with the whiskey and oak. That is an interesting idea about the crush, I've heard others complain about NB's crush off and on. Since it was only my second AG batch I didn't pay it much mind so I couldn't comment on the quality. I think the largest factor in my case was fly sparging at too high of a rate.
I made a Sierra Nevada knock off on Sunday and actually timed the flow rate to ~1qt/min and hit the pre-boil gravity dead on. However, for that particular recipe it was only 1.040 with a OG of 1.052. Incidentally, I did find quite a few grains that didn't have fully cracked hulls which I gather is less than ideal but I haven't been through enough grain to evaluate the quality of crush yet.