Partial Mash OG

Have done two partial mash American Wheat's using the Beersmith formula.  Mashing 5# of grain, temp and time and sparge spot on. Water usuage was incredibly close to the formula. Adding 3.15# of LME at 15 minutes before flameout. Boil size 3.7 gallons.  Equipment profile set for 4 gallon. partial mash/extract. Original OG per formula was 1.050.  Actual (compensated for temp) was 1.035.

Grains
2# 2row
2# 5oz White Wheat Malt
6 oz Munich
5 oz Crystal 10L
3.15# LME

Is the Formula accurate for partial mash? I have done partial mash kits that had 5# of grain and 4# of LME and always hit the target OG.

When I got started with Partial Mash I had the same issues a few times, after speaking with several more experienced all-grainers the topic of discussion was efficiency of the mash and the resulting lower OG. They suggested I keep a few lbs of DME on hand, take a pre-boil gravity reading and see where its at, if it looks lower than it should you can add some DME to the boil to compensate without really altering the beer too much. I have tried that and it works quite well.