These theoretical values are all very well in an ideal world. Surfacing is not the same as sliding. The suface cutting speed while sliding is constant on a cylinder for any one cut; for surfacing the same value reduces to zero as one approaches the centre point.
Young’s modulus, Hooke’s Law and other mathematical calculations are required to precisely affix a value for deflection of a cutter at any time during a cut – sliding or surfacing.
The likely cause for the poor finish can be attributed to any one of several reasons, depending on the user’s knowledge or guesswork.
It is most likely due to deflection of some kind. Along the surface or oscillations up and down while cutting.
These are often caused, in practice, by the cutting tip geometry altering in either a random or cyclic manner. Or both at the same time.
Tool quality and machine rigidity are two main variables – the others are represented in tables which offer speeds for different materials (workpiece and cutting edge), which are good practical targets or limits of sensible operation.
Hobbyists generally use small cutting tools in small, less rigid lathes, and sometimes are forced to use quite extended cutter overhang. These factors will alter that tool setting by a variable small amount. If the cutter is resonating, as it cuts, it will leave a less than ideal surface finish. Chatter is the phenomonon noticed, when the resonance becomes very obvious to the operator.
My advice to the OP is to minimise any possibilities of tool deflection, adjust the tool for optimum position and try at different speeds and feeds (so resonance can be avoided, or at least minimised).
That means avoiding cheap tools or holders (they bend more), use appropriate sharp cutters or inserts, ensure inserts are held securely in position on the holder (if used), shorten cutter overhang to a minimum – not just tool extension from the toolpost, but also to maintain the cutting point as close to the centre line of the carriage and toolpost, tighten all possible opportunities of unwanted movement in all planes (including the axis for the cutting operation over the desired range of movement). This may mean restricting the use of the slide at the extremes of travel.
There is little point in setting the cutter carefully at precisely centre height if the tool is immediately deflected downwards when making a cut. How many lock the compound slide and cross feed gib when making a sliding cut, or lock down the carriage and compound for a facing cut? Agreed, centre height is the ideal – but don’t be fooled into thinking a static setting is going to be the same as when actually making a cut with a non-rigid set up. It may be only the standard starting point for any particular machine and direction of cut.
I expect the OP’s operational shortcomings are actually a combination of factors, not simply – or only – cutter height. A ‘posh’ but poor QCTP could make things worse if it is excessive tool-overhang which is causing the grief! Lantern-type single tool holders certainly helped the modellers, of long gone days, to minimise some of the potential pitfalls while turning.
First thing to do could be to inspect the suface finish and see if it can be determined whether the roughness is down to oscilations around the circumference or along the surface. If neither of these, it could well be the cutter height (that typical eight degree magical nominal cutter-clearance angle in any or all planes).
If you’ve read all this, you may realise that surface finish shortcomings may be caused by any one (or more) of several, possibly small, sources – not just the tool height setting, which is just one of the possible causes.