Seat tube length is a tricky one. The most useful description for geometry calculations is center to center: from the center of the crank axle to the center of the top tube (C-C). Another measure is center to top (C-T) which goes all the way to the top of the seat tube including the part extending above the top tube.
For comparisons between frames of the same "size", it is useful to consider an "effective" C-C, which goes to a virtual horizontal top tube. I would call this measurement C-V, where "V" is virtual toptube.) It would be the length of the seat tube AS IF it extended up to a horizontal top tube. Eg: A trek 520 has a downsloping top tube, so to compare it to a frame with a level top tube, you would compare the "effective" seat tube.
From a geometry viewpoint, you want to represent the frame as a series of straight lines, so you would use use center to center measurements. So in that case, the seat tube would be center of the crank axle to center of the top tube.
Official Definition from BikeExchange.com: "Seat tube length is measured from either the centre of the bottom bracket to where the top tube and seat tube meet (Centre-Centre or C-C) or to the top of the seat tube (Centre-Top or C-T). Effective seat tube length is another phrase you may encounter which is similar to 'stack', measuring the distance from the centre of the bottom bracket to where a virtual horizontal top tube would meet the seat tube. "
Rather than having numerous Seat Tube fields (C-C, C-T and C-V) I rely on Stack and Reach to match frame sizes.