I assume you mean to get an otherwise equivalent glasses Rx, but without any astigmatism correction? If so, yes, it is quite easy to determine the “spherical equivalent” Rx. You take ½ the amount of cylinder (astigmatism correction) and add it to the spherical component (the first number)
So, with your right eye, -2.50 +(1/2 of -0.75) = -2.85 D. And in the left eye: -2.25 + (1/2 x -0.75) = -2.58 D.
_Written by J. Trevor Woodhams, M.D. – Chief of Surgery, Woodhams Eye Clinic