Post your recipe and I'll check it for you.
Or download brewsmith/promash or use www.recipator.com
and let that work it out for you.
If your grain has a ppg of 1040 then you have 15.5*40/6.5
620 ppg/6.5 = potential of 95
You ended up with 60
60/90*100= 66% eff.
Mash effieciency depend on the volume preboil. if you had 7.5g at 1046 then
Don't forget the volume makes a big difference. Most potentials are stated for 5 gallon brews.
Any reason you're brewing 6.5 gallon(US) brews? That's standard for the UK but not that normal for the US.