the area u need to focus is near to mid. but u hv to make ur 1st pair then u know. eg the angle fr near to mid to far.
for far is only see straight.
but when Reading book, see thing, it will be like near to mid that why this angle is important.
u can make for easy to read book or see computer monitor,
and one thing for sure do not make too small lens. it will be difficult to focus between near to mid.
if u hv $$$ good to hv enhance feature, but not too basic. u will find a lot of limitation.
one more thing is the lens center point must accurate. eye will pain not bcos of new glass but the center point is out.