how to calculate quantity for post order Feature

API Spot/Margin API
1

I have 100 USDT and I want to place a buy order:
quantity= balance/price
request_client.post_order(symbol=‘BTCUSDT’,side=Client.SIDE_BUY,ordertype=OrderType.MARKET,quantity=quantity)
I always get: Precision is over the maximum defined for this asset

2

You have to check the exchangeInfo endpoint (Binance API Documentation) → PRICE_FILTER → tickSize and ensure that you follow the precision.