With compact frames and the different configurations of mountain bike frames and lack of standardisation between manufacturers seat tube length is something of an irrelevance and stand over height is a better general guide. The more important measurement however is top tube length which because of sloping top tubes becomes "virtual" top tube length ie the notional distance parallel to the ground between the head tube/stem and the seat post. A rule of thumb for this measurement being correct is that sitting on the saddle, hands on handlebars if one looks down at the front wheel hub it should be directly under the handlebar ie not clearly visible. There are other factors affecting this measurement such as the reach of brake levers and some fine tuning can be achieved with stem length BUT it is better to start with a correct length of tube and keep the stem proportional to size of bike. This measurement will also be affected by how low you are able to ride and recent trends in bike design epitomised by Specialised Roubaix and Trek "performance fit" have made head tubes longer to give a more upright position more suited to the level of flexibility of real world cyclists as opposed to simply trying to imitate elite riders.