SNP testing is a critical step for anyone in the Sinclair / St Clair DNA study, and every other family out there. A single-nucleotide polymorphism is a very stable mutation and, therefore, one that can be considered irrefutable. If other in the FTDNA database have the same SNP, then it is certain that you share a common ancestor with them in a particular timeframe. That timeframe depends on which SNP it is that you share.
For instance, I also show the L21 SNP. The timeframe for the common ancestor in that one is quite old. Downstream (or more recently), I show the L513 SNP. This one is believed to have a common ancestor in Southwest England. The timeframe is somewhat uncertain. The geography of these SNPs works like this -
L21 - Western Europe, about 4,000 years ago
L513 - Southwestern England - ??
L193 - Possibly in the Borders Region of Scotland - about 900-1,000 years ago.
These timings are still being debated, as are the specifics of the geography.
Learn about our approach in exploring the L193 Lineage of the Sinclair DNA study here.