Do plus lenses cancel out minus lenses? For example, is wearing +1.50 reading glasses over -5.00 contact lenses the same as wearing -3.50?
Pretty much, yes.
That is why nearsighted people often grow dissatisfied with their vision from contacts as they reach their presbyopic middle age years. They find their near vision stays quite good (when not wearing glasses or contacts full time) but now they have to wear both glasses (for reading) on top- of the contact lenses. If they switch to single vision glasses, they can often just look under them to read and keep the good distance vision in both eyes.
There are other options with contact lenses (such as monovision), but what you have noticed is exactly right.
_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic