I was going through a discussion thread over at duolingo ("How to use El and La in Spanish"), when I came upon the following question:

Anyone know the percentage of Spanish nouns ending in -a that are not feminine? 5%? 10%? Or is it just 1%?

It made me wonder if someone might have answered this question here at Spanish StackExchange, but after going through several pages, the closest answer I found (punctuation edited) appeared as follows

... 95% of the cases, female gender ends with an -a ...

in the following thread ("Beyond memorisation and time, how can you master Grammatical Gender in Spanish? [closed].")

From this, I could surmise that 5% of nouns ending in -a are masculine. However, since the user who wrote that doesn't cite any source, I have to wonder if that is just a rough guess. Is there anything more scientific that someone could provide on the percentage of nouns ending in -a that are masculine and the percentage ending in -o that are feminine?