Aerocal

Ok.I think Im understanding that you already set it on a switch/Flightmode with a fixed amount of offset and this didnt work for you? You want a variable offset on the slider to manage the ele position at launch? It could be done but Im honestly thinking it could cause alot more complexity and potential problem than what you think your trying to achieve.Why not manage it with the ele stick if you want it to be variable?
You said that an experienced flyer didnt have trouble.It kind of sounds like you need to stick to a Flightmode based offset if needed and simply practice more.I have a half dozen hand,winch,bungee launch models and I only have 2 that a little up ele offset helps alot.
Im assuming you have it setup in Sail mode.IMO its going to be alot better in the long run to learn how to take advantage of Flightmodes and fixed basic values.Manage the slight variations with the stick themselves.It makes things alot simpler.You may have to fiddle with the launch values to get it where its comfortable for you but once you do that it should basically be the correct position for every launch and your brain will be knowing exactly what its going to expect everytime after a little practice.