There's plenty of data for 170-173-grain gas-checked bullets in various manuals. Use the starting loads listed and work up from there.
If none of your manuals show data for that weight range, tell us what powders you have, and we can provide data. Cast boolits, even with gas checks, are pretty forgiving as long as you don't try to push them to hard. Do you want to hunt with them, or just find a good all-purpose load?