We have:
Currency to trade: GBPCHF;
Account: 1000 EUR;
Lot size: 100000;
Leverage: 1 : 400;
Exchange rate EURCHF: 1.26000;
Current price GBPCHF: 1.65515;
Entry size: 3% from the account equity;

Calculations:
Target Margin = 3% of 1000 = 30 EUR;
Target Amount in EUR = Margin * Leverage = 30 * 400 = 12000 EUR;
Since we trade GBPCHF, we calculate the target amount in GBP:
Target Amount in GBP = Amount in EUR * EURCHF / GBPCHF = 12000 * 1.26000 / 1.65515 = 9135.12 GBP
Now we have to round that amount considering that Min Lot = 0.01 and Lot Step = 0.01;
So, min amount we can trade is Lot Size * Min Lot = 100000 * 0.01 = 1000GBP;
Rounded trade amount is 9000 GBP or 0.09 lots.
The Required Margin for 9000 GBP is:
Required Margin in GBP = Amount / Leverage = 9000 / 400 = 22.5 GBP;
Since our account is in EUR, we have to calculate the Required Margin in EUR:
Required Margin in EUR = Required Margin in GBP * GBPCHF / EURCHF = 22.5 * 1.65515 / 1.26 = 29.56 EUR

Final result:
Traded Amount: 9000GBP, Required Margin: 29.56 EUR

I don't understand  how my strategy (it's run on GBPCHF)  calculate the enter of position:
the parameters are : max lot 50 (1 lot=100 000 ) , trade 3% of the account, signal behavior (same, opposite) nothing, stop loss 50 Take profit 35
Account settings EURO, Capital 1000, leverage 400 and account exchange 1,2600chf
Datas are based in 5 DIGITS

It makes first SHORT trade at 1,65515 for a amount of 38 000, why this amount?
Anyone can ligth me in this problem, thanks for your answers.