The formula for calculating the Required Margin is fixed in FSB v2.57.11.0.

Lets see how it works in your case:

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