If you needed down trim... you need to shim the TE of the wing up.

I use 1/32 inch increments mostly... the first shim I would use in your case would be 1/16, as I would expect it to need at least that much.

Many people use 1/16 as thier test incriments then do the 1/32 or even 1/64th to fine tune.

If you are shimming slowly... you have less chance of making thengs very bad very fast.