My question is: Lets say 35mm is 40% scale what would I need to add to the 35mm to make it 50% scale?

I think it's a simple proportion (if I understand your question correctly).

\frac {35\, mm}{40%} = \frac {x}{50%

Solve for x \left( x= \frac {50 \,\times\, 35\,mm}{40} = 47.35\,mm \right)

Actually, the answer is 43.75 mm. A simple mistyping error - easy to miss.

A little dyslexia never hurt anyone (much)