Make yourself two blocks which are the same height as your scales off the ground. Put 2 blocks under any two wheels and scales under third wheel and note weight. Exchange scales and one block and note weight, repeat this for last wheel. Total model weight is weight1 + weight2 + weight3, lets call this Wm.

Measure carefully the distance from where the main wheels and the nose wheel touch the ground, lets call this L1. Measure carefully along the ground the distance forward from where the main wheels touch the ground to the desired CG, lets call this L2.

Now the total moment trying to rotate the model around the main wheels is Wm x L2, and the resisting moment from the nosewheel is weight on it, lets call this Wn times the distance from the mains to the nosewheel, L1. So the model will balance at our desired position when:

Wn= Wm x L2/L1. To be clear, that's Wm times L2 divided by L1.

So now we put the scales under the nosewheel and the blocks under the mains and move everything around until the scales read Wn and the model will be balanced at the correct point. If you need to add weight to the nose to get the correct reading on the scales you should really add the amount to Wm and re-calculate but in practice the additional weight will be small in relations to overall model weight so not really necessary.

Hope this helps,

Malcolm