how is the threshold for participation calculated?

Hi, how is the threshold for participation calculated? From the sip-007 we know that it is related to the total amount of the participation,

“25% If fewer than 0.25 * STX_LIQUID_SUPPLY STX participate in a reward cycle, participant wallets controlling x STX may include floor(x / (0.0000625*STX_LIQUID_SUPPLY)) addresses in the reward set. That is, the minimum participation threshold is 1/16,000th of the liquid supply.
25%-100% If between 0.25 * STX_LIQUID_SUPPLY and 1.0 * STX_LIQUID_SUPPLY STX participate in a reward cycle, the reward threshold is optimized in order to maximize the number of slots that are filled. That is, the minimum threshold T for participation will be roughly 1/4,000th of the participating STX (adjusted in increments of 10,000 STX). Participant wallets controlling x STX may include floor(x / T) addresses in the reward set.”

This total amount can be calculated by the end the participation period, which is before the prepare period. But when I try to participate in the reward cycle, the total amount cannot be calculated. So how is the threshold for participation calculated?