I think 40mm is just about right, you can get a fairly low mount that still allows you to see over the front sight base but is not so low to be uncomfortable to get down on. 50 should be ok too. I use a 3-9x40 Nikon Prostaff that works out well on my 5.56.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek