Find optimal percent

NoaHomeRun.... I believe that will depend on the %portfolio allocation of each asset, which you did not provide.

Consider two arbitrary guesses (not necessarily optimum), based on random %portfolio allocations: (1) sell at avg instead of min highest%; and (2) sell at min instead of avg highest%.  See the "avg-v-min" worksheet in the attached Excel file.

Note: Check my logic under "sell%" (column C) to be sure that I interpreted your rules correctly.

I would use Solver to find an optimal sell%, based on a random %portfolio allocation.  (Of course, you would substitute the real %allocations.)  See the "opt" worksheet.

For example:

Solver set-up, starting with empty B1:

Set objective: E13

To: Max

By changing: B1

Uncheck "make unconstrained variables non-negative"

Solve method: GRG Nonlinear

PS.... The value derived in B1 is __not__ rounded to any particular precision.  I would __not__ try to force Solver to derive a rounded value; often, that causes Solver to fail because of internal "discontinuities".  Instead, I would round the value myself (e.g. to 6 decimal places) __after__ Solver finds solution.  Arguably, the rounded value might no longer be the best solution.

PPS.... I have never been confident in Solver to find a truly min or max value, even after adjusting the constraint and convergence precision options. IIRC, sometimes Solver finds very different min or max values, even when starting with the same initial value (e.g. empty B1). Arguably, the fault might be my rudimentary knowledge of Solver.  "A word to the wise ...."