There are typically two different tech levels you would like to obtain in a set:

(1) Mid-level tech during growth
(2) High-level tech during stockpiling

The general concepts are (as you noticed) for

(1) the higher amount of tech you have, the harder it is to increase the tech %, thus the money spent should be balanced between land growth, defense, and tech. (Each turn you can get away with more tech without loss in land growth or defense maximizes your production efficiency)

(2) you want the highest amount of tech you can get, such that at the end of your stockpile period, you make more than had you not bought the tech.

For example, assume your stockpile period is 500 turns using your example (5mil income, 167% bus/res on 11k acres). 10% increase in bus/res is equivalent to about 1mil extra per turn (5*1.1*1.1). Let each tech point cost $2k.

Then not buying tech points, you would a total income of
5*500 = 2500mil

Buying 112k (x2) tech points, you would obtain a total income of
6*500 - 448 = 2552mil

To be more exact, you could include food costs and the gain in income from reselling the bought tech at the end for $1k, which makes buying more tech the better choice.

Note, another key benefit is avoiding increased expenses esp when you start going over 2bil stockpiled, stockpiling tech may boost your income and net you more income (if tech is super cheap) compared to stockpiling food.

Hope that helps.