You can build a teardrop with any tongue weight you want, if you put the wheels in the right place. It's perfectly possible to build a teardrop with a zero tongue weight, though it would be dangerously unstable to tow.
My trailer balance spreadsheet (Design Library link above, then look for Trailer Balance) can help you if you can estimate the weights of the bits of your trailer. Failing that, putting the axle at 40% of the body length from the back of the body will give you around the 15% tongue weight you are asking for.
Be aware that in Britain a homebuilt trailer completed after October this year will require to pass a trailer IVA inspection before it can legally be used on the road. This isn't a huge deal but does have some bureaucratic implications. See the
trailer IVA manual and the
guide to IVA. I am just working at the moment on producing some stuff to help British trailer builders pass the IVA and will post those on the
British teardrop forum and probably here too.