Measuring for a new jib or head sail is easy. A high cut clew on a head sail allows for good visibility from the helm position. Good visibility when cruising is worth the small performance loss of a bigger sail. If the clue and tack form a horizontal line with the deck you get a nice looking sail that you can see under.
First determine how high off the deck you would like the tack and the clue to be. From the helm point you will want to be able to see under it easily. If you attach a measuring tape to the shackle of the jib halyard (and a line so you can get the halyard back down) and run it up the fore-stay, you can get the length of the luff side of the sail from the top of the fore-stay to the point above the deck you have picked. If you are measuring for a roller reefing sail the top or upper swivel needs to be close to the top but low enough it does not bind and let the halyard wrap around the top of the sail when it roles in. You also have to consider the tack point on the lower drum.
You need to consider how long your genoa tracks are and where they are located when you pick how long you make the foot of the sail. A 100% jib will not extend beyond the shrouds, where as a 150% jib extends way past the shrouds. The length of the leach is determined by the length of the foot of the sail. By making a diagram of the sail with the dimensions of the foot side and luff side on it, and measuring the angle of the fore stay the foot side of the sail the leach dimension can be calculated. This is done by marking off right angles on the diagram on the foot and the luff sides and then use the Pythagorean Theorem (The theorem that the sum of the squares of the lengths of the sides of a right triangle is equal to the square of the length of the hypotenuse.) to calculate the length of the leech. See diagram below.
If you give your sail-maker more measurements than he needs he can pick what he needs to make the sail and can give suggestions on what can be done.