I'm guessing that the 1.5 listing is for a nominal force. You could try a few tenths above and below that without much worry, say 1.3 to 1.7. But you may want to just leave at 1.5 while you dial in VTA. Just remeber that a few small changes in VTA will not affect VTF too much, but more than that and you should re-check VTF.
Another trick I used on the MMF 7, and on my Basis Vector, to help adjust VTA is to use playing cards as shims. The Project has two set screws holding it so you can shim one side, tighten the screw on the other side, then remove the cards and tighten the remaining screw. There are varying opinions on how tight the set screws should be. Personally, I don't like to tighten them anymore than it takes to keep the arm in place. Snug and no more. Others have reported that they get better bass with the screws tighter.
Another trick I used on the MMF 7, and on my Basis Vector, to help adjust VTA is to use playing cards as shims. The Project has two set screws holding it so you can shim one side, tighten the screw on the other side, then remove the cards and tighten the remaining screw. There are varying opinions on how tight the set screws should be. Personally, I don't like to tighten them anymore than it takes to keep the arm in place. Snug and no more. Others have reported that they get better bass with the screws tighter.