Further speculation. If Margaret died in 1990 aged 93, it may well be that her daughter Moira has died in the 30 years since then.
There are seven deaths of Moiras with mother Macpherson in the SP index. Two can be discounted because they are too young (born when Margaret Macpherson was 49 or more).
That leaves five.
Moira Macdonald or Dickson, died 2000 aged 72, so born 1927/1928. Moira Macdonald was born in Barrhead and Levern in 1927. Moira Macdonald married John Morrison Dickson in Barrhead and Levern in 1948.
Moira Isabel Millar or Mackenzie, died 2006 aged 75, so born 1930/1931. Moira Isabel Mackenzie was born in Forres in 1930. Moira Isabel Mackenzie married Alexander Millar in Forres in 1958.
Cecilia Moira B Archibald or McKandie, died 2012 aged 81, so born 1930/1931. Cecilia M B McKandie was born in Rothiemay in 1930. Cecilia M B McKandie married Ian Gilbert Archibald in 1956 in Towie.
Moira Kay Gibson or Cleland or Webster or Thomas died 2020 aged 85 so born 1934/1935. Mary Kay R Webster was born in Renfrew in 1934. Mary Kay R Webster married James Watson Cleland in Renfrew in 1954. Moira Kay Reid Cleland married Daniel Thomas in Renfrew in 1997. Moira Kay Reid Thomas or Webster married James Gibson in Renfrew in 2001.
Moira Jack or Sutherland, died 2019 aged 82 so born 1936/1937. Moira Sutherland married Alexander Watt Jack in Duffus in 1959.
It's unlikely that Moira Isabel Millar, Cecilia Moira McKandie of Moira Sutherland are yours, because they are all from Moray/Banff/Aberdeen.
So that leaves Macdonald and Webster.
There are no marriages of a Ma*g* M*cpherson to a Webster between 1911 and 1930, but there are 5 marriages of Ma*g* M*cpherson to a M*cdonald. I think the ones in Rosskeen and Boharm are unlikely, but you might like to take a punt at one or more of the other marriages to see if it's your Margaret. Or maybe one of the surnames might ring a bell?
I suppose that to be thorough I should repeat the exercise using McPherson instead of Macpherson, but not tonight as sleep beckons
And of course Moira could still be alive, or she might have left Scotland, in which case all of the foregoing is pointless.