If you are satisfied that the following marriage relates to your ancestors:
Duncan Stewart in Bohally, Dull parish, and Christian Robertson in Tressait, married 22 Jan 1800
then Christian Robertson probably was the youngest daughter of Duncan Robertson and Susan McDonald, who were both living at Grennich when they married December 1754
This was the only Robertson family living at Tressait at this time as far as we are aware.
We have baptisms for 7 children in the Blair Atholl OPR
Duncan Robertson Susan McDonald Grennich Elspeth 30 Sep 1755
Duncan Robertson Susan McDonald Balnabodach Janett 2 Aug 1758
Duncan Robertson Susan McDonald Over Bohespic Margaret 25 Aug 1761
Duncan Robertson Susan McDonald Grennich Alexander 19 Dec 1764
Duncan Robertson Susan McDonald Grennich Elspeth 16 Aug 1768
Duncan Robertson Susan McDonald Grennich Beatrice 11 Jul 1772
Duncan Robertson Susan McDonald Tressait
in Strathtummel Christian 10 Apr 1778
Assuming that this the correct girl she would have been 22 years old when she married, which is reasonable.
The family seems to have moved around a bit to start off with, suggesting that Duncan was not a tenant farmer - more likely to be a farm servant or artisan (weaver, tailor, shoemaker?)
We think that did not move from Tressait after 1778 as there are marriages etc for other Robertsons who would appear to be Christian's older siblings.
Donald Robertson in Gaskan of Bohespic and Janet Robertson in Tressait illegitimate daughter Ann. 7 Aug 1780
John Stewart in Bohally, Dull parish, and Elspeth Robertson in Tressait, married 3 Feb 1789.
Donald Cumming in Riechail and Margaret Robertson in Tressait, married 15 Jan 1791.
Alexander Robertson in Tressait and Isabel McIntyre in Blairbuie illegitimate son Duncan
22 Mar 1793
Patrick Gow in Achinruie and Betty Robertson in Tressait, married 6 Jan 1797
In this case Betty is not a familiar name for Elizabeth / Elspeth, it would have been pronounced Beattie (i.e. Beatrice)
(Their 2nd son Duncan, named after his maternal grandfather was the Sulky Bārd, a renowned Gaelic poet)
Alexander Robertson in Tressait and Cecil Cummings in Carie, Logierait parish, married 27 Jul 1799
Alexander Robertson and Cicel Cuming in Tressait daughter Susan 7 Jul 1800
If you consider tracing back from Susan McDonald keep an open mind. There was a Margaret McDonald, married to Donald Stewart in Grennich between 1752 and 1756, but she also had the alias name of Caldel, so she probably originated from Glen Fincastle.
Throth