@ Morag, by the mid 19th century thirlage had been abolished in Scotland. The centuries-old system whereby the tenants of an estate were compelled to use that laird's mill & no other, plus supply so many days free-labour to maintain the mill, it's water supply & cut peats for the kiln, so not always a popular figure, but an independent one. The miller was a tacksman & held a lease, he was a skilled man & generally did much of the mill-wright's work himself. Meal was used as a currency & paid wages, rents, debts etc! The fee (multure) charged by the miller for grinding the corn (oats & barley) was paid in meal, (the percentage varied,) this the miller sold at the market which required a horse & a croft to support it. Millcroft/Milncroft are common place-names!
An asset to the estate, the miller paid a high & steady rent, he was also used by the laird for deducting meal from farmers to pay their rents & often consulted on lettings & rentals.
The miller had an assistant who also was paid in meal, often a son who hoped to follow his father but might have to find himself a mill elsewhere.
Before your man's time the parish also included Ardoch, try a search of the Muthill Horse Tax Rolls, Dog & Cart, taxes etc, levied to raise cash to fight the French. The older Hearth Tax also!
A quick check of "Old Scottish!" doesn't show many Stewarts!
Skoosh.