The right amount of compression to use depends on the vocalist and mic, and even then is fairly subjective
When you compress the dynamics of a sound you get a more consistent volume, and you can make the overall volume louder without clipping. You also get less dynamics. Depending on your style, you can go for a heavily compressed sound like ZZ Top (in which case you need to be careful with monitors using the DL), or no compression like JJ Cale, or anything in between. Sometimes when you have a singer who doesnt have very good mic control, then it can be a useful tool for keeping the vocals more consistently in the mix
I find that the default male vocal compression settings work fairly well for me. I'm not much of a singer, but I do lead voc on a couple songs, using an AKG D3900. My mic control is not great, so I lower the threshold a bit, to about -35db, with a 2:1 ratio. For me, its more about evening out my vocals, rather than going for a particular 'sound'. I play with and occasionally do sound for some really good vocalists, one of which is adamant about no compression and the others dont really care so I use the defaults and EV757 voc mics most of the time (I realize I'm dating myself here, both in terms of gear and musical points of reference).
As a point of reference - RR is using a higher threshold and gain, which is good for someone with better mic control who still wants to make sure that the input doesnt clip if he's eating the mic on load passages. In this case, its working more like a limiter than a compressor
If you are going for a particular sound, then you might want to spend some time tweaking the compression for your mics and vocal style. However if you are doing a gig tonight with all new equipment, I wouldnt worry about the compression settings too much, so go with defaults unless you have someone in the band with really poor mic control or if you have a punk rock screamer or something like that in which case you probably want to drop the threshold and maybe increase the ratio a bit.
Bottom line - the right compression setting are a bit like asking what is the best amp for getting overdrive guitar tones. You may get a lot of good opinions, but no right answer. If you have a particular style in mind, then you can get some opinions that will be more useful. I'd give some extra weight to your sound man friend's opinion, as presumably he has a good idea of your specific style, equipment and vocal abilities.
