If an upper limit is known in advance, simply SetLength(arry, 210); at the outset.
Keep a count of the number of indexes allocated, and when all indexing is complete, call SetLength(arry, count).
Of course, the difference in speed compared to multiple calls to SetLength() may be negligible. Timing the two alternative implementations is the only way to know if there is any significant speed gain. It is surprising how often optimisations which in theory would be expected to make a difference in practice make little difference, or even have a negative effect with certain data. Compilers are getting ever more sophisticated, and low level caching/pipelining etc. over which the high level programmer has little or no control may ultimately prove to be the determinative factor.