To be clearer, I should have written 'a calculation' and 'a computation', as in the examples of the older usage, 'compute' is a singular noun, referring to a sp

To be clearer, I should have written 'a calculation' and 'a computation', as in the examples of the older usage, 'compute' is a singular noun, referring to a specific calculation, and the acceptability of substituting, under current usage, 'compute' for 'computation' here (where the latter is shorthand for computational work or effort, rather than referring to a specific calculation), has no bearing on the usage five centuries ago - and nor does the usage five centuries ago have any particular relevance for today, given how much computation has changed since then.