ThomD
>Main thing is where it is dished. If it is hollow right in front of the mouth, or high just behind, and they often are, then you have a problem. Other than that I think the try it advice is pretty good. Getting it closer over a full length is not a job for shop methods. As a general issue getting it much flater is a job for a surface grinder. You can even out "texture", ups and downs, but really nailing it all the way takes an SG. Most woodshops don't even have the gear to measure that accurately. Most plane will distort 5 thou in use form hand pressure. Still the flater the better, but you may try it and find it does everything you need it to.

In general, I don't totally feel woodworking and metalworking are all that different in tolerances, but where they apply those stanadards is different. You want to bore a hole that is a press fit for a bearing it has to be just so, When woodworkes make parts like that they are normally oversize. But woodworking joinery, or the steps in which woodworker's reduce a surface for joints or finish are right in the way of the same tolerances metal workers use, so there is more in comon than many allow.