I've been in this exact situation before! The easiest solution I found is using a dedicated cryptocurrency calculator. For Tron specifically, I always use
https://paybis.com/tron-calculator/ - it's super straightforward. You just enter the USD amount ($300 in your case) and it instantly shows you the live TRX equivalent. The rates update in real-time, so you get the most accurate conversion. What's really helpful is that it also shows the current TRX price separately, so you can double-check everything before making the payment.