Amarillo Blonde (Sub in Munich?)

I want to try out my new 2 gallon cooler for my first partial mash. So, I was going to brew another batch of the Amarillo Blonde I just finished and just sub in a pound of 2 Row. But, as I was digging through my grains, I found a pound of Best Malz German Munich (6-8L).

I'm assuming that adding the Munich would give it a maltier, biscuit type flavor, which is fine. It would also allow me to do a somewhat larger mash, and I assume a larger mash is going to help hold my temperature steady longer. (I'd be using ~ 4 Q Water/2.5 Lbs Grain rather than say 2.5 Q/1.5LB.) I suppose I could also just run out and grab more 2-Row.

Toss in the Munich, grab some more 2-Row, or just go with the super small mash? Thoughts?

Batch Size: 3.5 gal
Boil Size: 4.5 gal

2.50 lb Extra Light Dry Extract (3.0 SRM) Dry Extract 62.50 %
1.00 lb Pale Malt (2 Row) US (2.0 SRM) Grain 25.00 %
0.25 lb Cara-Pils/Dextrine (2.0 SRM) Grain 6.25 %
0.25 lb Caramel/Crystal Malt - 20L (20.0 SRM) Grain 6.25 %
0.13 oz Columbus (Tomahawk) [14.40 %] (60 min) Hops 10.3 IBU
0.16 oz Amarillo Gold [8.40 %] (10 min) Hops 2.6 IBU
0.36 oz Amarillo Gold [8.40 %] (5 min) Hops 3.3 IBU
0.50 oz Amarillo Gold [8.40 %] (0 min) Hops -
4.5 gal Poland Spring (R) Water
1 Pkgs Nottingham Yeast (Lallemand #-) Yeast-Ale

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Add the munich! One of my favorite grains. However, make sure you pay attention to your hops (IBUs) if you are adjusting your OG.

I brewed the recipe above with the extra pound of Munich. So it looked something like this:

2.50 lb Extra Light Dry Extract (3.0 SRM) Dry Extract 50.00 %
1.00 lb Munich Malt (9.0 SRM) Grain 20.00 %
1.00 lb Pale Malt (2 Row) US (2.0 SRM) Grain 20.00 %
0.25 lb Cara-Pils/Dextrine (2.0 SRM) Grain 5.00 %
0.25 lb Caramel/Crystal Malt - 20L (20.0 SRM) Grain 5.00 %
0.13 oz Columbus (Tomahawk) [14.40 %] (60 min) Hops 9.0 IBU
0.25 oz Amarillo Gold [8.40 %] (10 min) Hops 3.7 IBU
0.25 oz Amarillo Gold [8.40 %] (5 min) Hops 2.0 IBU
0.50 oz Amarillo Gold [8.40 %] (0 min) Hops -
4.25 gal Poland Spring (R) Water
1 Pkgs Nottingham Yeast (Lallemand #-) Yeast-Ale

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Beer Smith projected an OG of 1.042 and I hit 1.034. Pretty poor efficiency, but it was my first try so I'm not too worried about it. I'd like some feedback on hitting the mark a little better.

The only thing I changed from the BS recipe was the sparge water. BS said to sparge with 2.79 gal of water. I read in another Partial Mash thread that you should sparge with 2 Quarts/Pound, and over-sparging can result in off flavors. Is the sparge water the reason I didn't hit my OG?

Relevant Info: I mashed in at 157 degrees. 60 minutes later the mash temp was 153. I used a 2 gallon cooler with a broccoli strainer and grain bag. After draining the mash and recirculating, I poured the sparge water (5 quarts/2.5 pounds grain) over and drained again. Maybe my high mash temps played a role?