How to calculate lot size by risk, checking min and max lot available to symbol?

I use this to calculate amount to trade, where EPC_lever = percentage Equity to leverage, & with a Fixed_Amount override

double amount = 0.001;
if (Fixed_Amount > 0) {amount = Fixed_Amount;}
else
if (EPC_lever > 0) {amount = (account.getEquity() / oneMill) * EPC_lever;}
if (amount < minAmt) amount = minAmt;
if (amount > maxAmt) amount = maxAmt;
// millions to trade in Base (Primary) currency
amount = new BigDecimal(amount).setScale(3, BigDecimal.ROUND_HALF_UP).doubleValue();

But if you want to calculate the % of your Equity to Risk then that's a different story ... what would you base that on?

BTW, there is a user interface added since the version 3.5.9 for this. We call it Amount calculator and it is available next to the amount spinner.