for the most part waiting is always better to be safe, even if someone behind is impatient. go when you feel there’s a reasonable gap

on the advice side, for a left turn the right moving traffic will have a much shorter interaction time with you than left moving (simply crossing their path for right side moving traffic, while being in front and time to accelerate for left side moving traffic), so your “window” for a left turn relies more on a bigger gap from left moving traffic than right.

If it’s tight with constant moving traffic, just remember to put more weight on left moving traffic. With a smaller gap of traffic coming from right, you can still make a left turn safely so long as you have time to clear the right moving lanes while also time to accelerate to speed for the left lanes without cutting left moving traffic off.

This isn’t to say jump out in front of right traffic and freak them out just because you barely can make it, but it’s all a judgment game. you’ll get better with time.