Maybe there is a tool or technique for this... it might save me lots of time if it exists.

I have this square (350x355 mm) and I want to draw some vertical cuts on it, 20 mm each. I want each dimension (a,b,c,and d) to be the same, and calculation is very straightforward but in case I change something, would need to change something by hand. For this drawing a=b=c=d is calculated by (350-3x20)/4 = 72.5 mm. However, should I change 20 mm to 15 mm this number would be different.

So, my question is: can I somehow have SW do this calculation for me ? By specifying a=b=c=d somehow ?

I know I could do this.... 

but what I would like to know if there is someway to specify dimensions to be the same without actually doing some external math. I hope my question makes sense.