Patrick,
Here’s how I would handle your prescription question:

1. The overall spherical power of a lens, is the sphere plus “half” of the cylinder power. In your case, that would equal -1.50 + 0.25 =
-1.25 spherical power for distance

2. Your patient likes his reading glass, which has a power of +0.75

3. The difference between his distance power (-1.25) and his reading power (+0.75) should be the power of his bifocal. Thus, your patient needs a +2.00 bifocal add.

4. I would write the final prescription as:
-1.50 +0.50 axis ?
Add: +2.00 bifocal

5. If you are putting in a progressive bifocal, you may want to add another quarter diopter to the bifocal (optional).

Good luck!