The distance from the jib tack to mast, roughly speaking, would indicate a 100% sail.
Similar to the diagram below.
If that distance is, say, 10ft on a given boat and you used a sail with an 11ft distance, that's 10% longer and would be indicated as a 110% jib. 14ft? that's a 140%.
If you're calling to order them would not waste any time researching who to buy sails from for a CP19. Call Dirk at National Sail. So many of us here have sails from National and agree they are the best value out there-really good quality at a really good price. However, you may feel more comfortable with a local sailmaker, and that's an option too.
Are you a lake or coastal sailor? Trailering or using a dock? There are options for sails to think about first before calling. Number of reefs? Bolt rope foot or loose footed? Short or long battens? Furler now or future? Cloth weight? And as you asked about, what percent jib to use.
I bought a 110% and 140% for use on a furler and a 170% nylon free flying drifter. The 110% has thicker sail cloth for higher winds usually early and late in the season while the 170% is nylon and extremely lightweight and for us in light air. I will normally use the 140% most of the season, and it shows as it's the most worn one. For the main it was loose footed, fully battened and of medium cloth weight. The full battens help with performance but are long and unwieldy. We use a dock so dropping and covering sail is very easy, but removing them each time if I trailered would be a pain, since you can't fold the sail up with them in place.