I'm guessing you mean pre-load on the rear shocks. It would help if you specified which model and if you mean stock Showa shocks. If there is just one adjustment available it is invariably pre-load. Very few bikes allow you to adjust ride (seat) height.
As to settings you can assume a mid setting is appropriate for an average sized Japanese rider so maybe 60kgs, but an original owners manual may give more info. Unfortunately it may be in Japanese! Hence why suspension improvements are often the first port of call for 400s.
Send me the page on suspension settings I have a Japanese sister in law who can translate it.
Either way you're likely to find the suspension soft so dial up the pre-load. Main cures are aftermarkets Ohlins or similar plus CBR600 fork springs or progressively wound jobbies. Either way you're looking at spending some wedge.
Cheap fixes are washers or heavier weight fork oil and playing with the level.