If you want the most accurate value, it will be different for every ISP due to various physical and configuration difference from one ISP to other. Start from 40 and build yourself up from there until you get a stable connection with respectable bufferbloat.
Speeds above 50-100Mbps you won't notice much throughput for pps difference so if you overestimate overhead it's fine and do not overshoot your ISP bandwidth limit, try to keep it atleast below 90-95%.