String spacing

Kitt

Hello

I'm thinking of putting together my first partscaster Strat. 

The body will probably be a Strat Deluxe model as these are readily available direct from Fender. The neck would be a 42.86mm nut width. 

Looking at bridge options, I want to ensure I get the correct string spacing. So how do you know what the correct spacing is and also how do you measure string spacing on an existing guitar? Would you use a ruler and measure middle of string to middle of string or calipers and measure the outside of one string to the outside of another? 

Thanks for your help and advice.