Calculate/estimate your expected 'sailing load' in pounds, including weight of crew. Then find your 'immersion' data "pounds per inch of immersion" and add that amount above your present waterline.

Such calcs. are found from boat tech. websites such as http://www.image-ination.com/sailcalc.html .... go to Part 1, select your boat in BOTH columns, hit enter and see POUNDS PER INCH (immersion) in the bottom 'results box'. Obviously such data is 'approx.', so add a bit of 'fudge factor' to be sure and so you dont have to do this 'twice'.

Dont forget to add barrier coat, etc. above the 'old' waterline and below the new bootstripe, .... good practice to also add barrier under the bootstripe. The price of 3M 'fine-line' tape has dropped, so making/masking a *precise* new water line and new bootstripe is now 'cheaper' than before.