To calculate appropriate wheel and tires, you need to calculate stock wheel/tire diameter so you have a baseline for maintaining speedometer and odometer accuracy.
Formula is:
Wheel diameter + (tire width*aspect ratio/100*2)/25.4.
Wheel diameter is in inches, tire width in millimeters. 25.4 converts the tire height from millimeters to inches. Aspect ratio is tire sidewall height divided by tire width. One tire's measured height and width don't necessarily match a different manufacturer's measured values for the same size tire, so exact comparison isn't possible, but it's close enough.
195/50-15 = 195 mm tire width, 50 aspect ratio, 15 in rim diameter.
Use a spreadsheet to generate a matrix of possible sizes and select those that deviate less than 2% from stock. E-mail me direct and I'll send you mine in Lotus 1-2-3 format as Excel sucks!.