An off-center table will have a direct affect on the value of the diamond due to loss of symmetry and salability.
With practice, the simple process of louping the diamond for estimation of table size will come without needing any measurements. In the buying process, a precise percentage will not be necessary to determine a fair price on a stone, in most cases. The most important aspect in estimating table size, is to determine the negatives and/or positives of the diamond in question. If the table is large, then you know the value must be discounted at least slightly. If it is too small (less than 53%), then perhaps you will consider the weight lost by your cutter in opening it to an acceptable size. If a table is severely off-center, the table size is probably the least of your problems in resale or re-cutting.