When tracking percentage of portfolio allocation, do you track the amount invested or the current value of the stock?

Current value. The point of tracking % allocation usually is to prevent exposing yourself too much to a single company/sector/industry/region and also to help you rebalance accordingly.

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For example, let's say you hold two ETFs in a portfolio of 20k, VOO and VXUS in a 60:40 ratio (12k VOO and 8k VXUS). After 1 year, the portfolio has grown to 25k and the ratio has skewed to 75:25 due to VOO outperforming (18.75k VOO and 6.25k VXUS). So to maintain the ratio, you should (theoretically) rebalance your portfolio back to 60:40 (15k VOO and 10k VXUS).

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This reduces your exposure to VOO since your original allocation was 60:40. It also kinda helps you buy more of the less high performing stocks/ETFs in your portfolio. In a sense this is like helping you to take profit on VOO and buy the dip on VXUS as well.