Says your particular organizations policies, why is the line drawn at 90%, why not 95%? Is this restriction based on calculations or was it simply arbitrarily chosen?
If it's arbitrary, allow me to offer an alternative, focus support only on "modern generation" browsers which auto-update by default. All popular browsers adopted auto-updates as a collective effort to improve end-user security, and so as web developers we're able to encourage this by only supporting those platforms.
As you can see on caniuse, grid support on those modern browser platforms is 100%. The only ones lacking are IE which microsoft has abandoned, and Opera Mini (I can't speak good or bad about that, I haven't examined it).
The idea that you must support as many browsers as possible is a relic of the 1990s dot com bubble when it was unclear which browser was best and there were no agreed upon standards. It's now 2018 and you're either using Firefox or Chrome and to a much smaller extent, Edge or Safari. Any other browser that wants to be taken seriously is going to support the standards set by those platforms.