There is a 3 page will on Scotlands people for your Thomas - cost £5 to download. Most will be legal jargon, but worth downloading anyway. I didn't check for any Chrystals.
Regards, Steve :O)
Craig Thomas 01/11/1865 warehouseman, residing in Craignestock Place, Glasgow, spouse of Elizabeth Chrystal Inventory Glasgow Sheriff Court Inventories SC36
From the IGI there is only one Sophia Porter born in Scotland from 1770 to 1790
ch 22 Nov 1787 in Coldingham, Berwick, Scotland - child of Thomas Porter and Margaret Martin - the OPR will be available on SP.
Sophia had at least one sister :
Elizabeth Porter ch: 16 May 1784 Coldingham, Berwick, Scotland - also an 'extracted' record.
This would appear to tie in with the record found by by daval57, in that a naming pattern is used - Margaret, first daughter, named after mothers mother. I'd now be looking for a John Chrystal born 1760 and 1795 with a mother whose name was Sarah.
I've checked the IGI - 1760 to 1800 - 39 John Chrystals born. None with mother Sarah - a handful with no mothers name.