When working with "effing gigantic" tables, I believe the rule of thumb should be to set as many slots as $sqrt(items) aka $calc(items^(1/2)). I have one table that approaches a million items, geoip address ranges, so 1000 is a better value for /hmake.
If you know for sure that your table will only contain 100 items, then set it to 10. If it'll only contain 50 items, then set it to 7 to save a few processor cycles.
It's certainly not that big of an issue, I just recommended -m[N] for convenience of coding. /hmake is almost -never- used unless you are loading a table from file, and for that /hload -m[N] could be made a convenient option.