Keep sparging or let it flow?

Another mash efficiency question:

I fly sparge using a zapap lauter tun, usually hitting 72-74% efficiency. For most batches, I fill the LT to the brim with grain, about 16.5#. My sparging method is to just keep pouring water on top of the grain bed until I collect 7gal in my brewpot.

Because I keep pouring on the water, there's always a bit of liquid left in the LT after I get my 7gal and shut off the flow. It's about 1.5-2.5 gal, frequently with gravity in the low 20s.

I'm wondering if I could improve my efficiency by doing a better job calculating the exact amount of sparge water I'll need. On the one hand, I feel like adding water to the top of the grain bed is helping to push the sugary mash water out the bottom. On the other hand, I may be diluting the LT so much that I leave a lot of sugars behind.

Thoughts?

If your gravity is still in the 20's then I would guess you could scale down your mash a little bit (unless you are trying to make a big beer, and don't mind sacificing efficiency to start with a higher pre-boil gravity). I think many people are willing to lauter down to 1.010'ish assuming the pH doesn't get too high.

The thing is, I've scaled down the mash but it doesn't improve my efficiency. I still get 72-74%, but now I have to make less beer because there aren't enough sugars to hit the target OG.

Hopefully someone with more grey matter can correct me, but doesn't having a target gravity already mean you are willing to settle for lower efficiency? If your first runnings are 1.090 and you sparge and lauter until you collect 6 gallons (with the last runnings at 1.030), then you might have 1.060 preboil, but you would likely have poor efficiency because there still would have been all the wort you could have collected by continuing to sparge/lauter/collect wort until the SG is almost 1.010. This will lower your pre boil SG though, which might not be desired if you want to make a high OG brew. There's other threads about it, but I think there are basically two options when trying to juggle boil gravity, boil volume and mash efficiency.
1. Go with a target pre boil gravity and volume, if the combination of these two with your grain bill means that the last bit of wort you collect is higher than ~1.010 then you might not have great efficiency.
2. Collect wort until your SG gets down to ~1.010. Depending on the grain bill, this could mean that you have to collect more wort than desired and have a very long boil if you want to shoot for a certain batch size/OG. This could also let you get efficiencies up into the 90%'s AFAIK.

I think a lot of people just go with the first option, since grain is cheap. All of this assumes that you are getting good conversion though, would it be worth it to do an iodine test?

I hear what you're saying, and this kinda leads to another issue I have that I'm sure a lot of other homebrewers experience: we're greedy. If a mash goes wrong and the gravity is a little lower, we have to make the decision between target gravity with less beer, or target volume with lower gravity. I'll straight up admit that I always end up choosing to have more beers.

But that doesn't change my current situation: I'm maxing out my LT with 16.5# of grain, and I'm maxing out my brewpot with 7gal of runoff. I'm wondering if my sparging method is leaving behind too much sugar because I dilute it so much.

I do an iodine test and conversion looks complete (though I'm considering buying a barley crusher so I can crush more finely - I suspect my LHBS has a coarser-than-necessary crush setting). And even if I had a bigger brewpot and could continue sparging, I don't think I want to get into 90min+ boils (60-75 is my standard) Finding 6 hours to brew is taxing enough on my schedule! I just need to figure out how to get more of those sugars from the LT to the brewpot, and I'm wondering if I should stop adding fresh sparge water so that the LT can (almost) completely drain itself of liquid.