In Martingale, the next profit should always be 2 times (or N times) the previous loss. So, if your position size is determined by some other factor, you can calculate your TP as:
PreviousLoss * N / (1LotPipValue * PositionSize)
For example, if your previous loss was $100, you want the next profit to be twice as large, the pip value is $10 (e.g., you trade EUR/USD), and the the lot size is 0.5, then TP = $100 * 2 / ($10 * 0.5) = 40 pips.
However, I strongly recommend against trading with any kind of Martingale strategy - it will bring you only ruin and grief.