As per title. For example I place an anchor on top of |O| and an anchor at the base by gravecomb. Now, there is a good practice or a custom thanks to which are the anchors at the right height? Also, the two anchors of the example it is advisable that both (base and mark) are placed at the same height, thus changing the position of the comb glyph if necessary, or they can be placed at different heights?
In a Latin font, it is quite common for above mark anchors on bases to be vertically aligned to or near the tops of letters, e.g. at the x-height and cap-height, and for below mark anchors to be aligned to or near the baseline. Then the corresponding anchors on the mark glyphs are set at appropriate distances below/above the mark outline to achieve the desired positioning in composites or in GPOS output.
It is also common in Latin fonts for the marks outlines to be aligned to each other at a default height, and optically positioned horizontally at the same location relative to the zero-width, such that all their anchors can be in the same location in the glyph space.