Is there a defacto guide for how many/how much gap between the bottom of the stem an the headset?
No, none at all.
Using saddle, bars & grips that I’m happy with already, I usually make sure seat height & fore/aft position is spot on first and that the stem length is giving about the right reach.
Then I tend to go for a few long rides to see how my weight is distributed – if my arse hurts, I take spacers out. If my wrists hurt, I add another one.
All a bit trial and error, but usually end up getting the balance about right.
Not very scientific, but it works for me 🙂