You want to add the exact amount you have now if you want 250/ml, as the powders already been accounted for.
so if you've got 10ml every 1ml is 500mg test so you'll still need to add 1ml each ml to make it 250mg so another 10ml in total.
One problem you could have, you may make it crash as you'll be halving the solvents, but I assume there so much solvent in it already to keep it from crashing at such high mg per ml that you'll be ok.