With proportional income tax, the multiplier formulas change because consumption rises by c(1-t) times the increase in income, not just c times. The government expenditure multiplier becomes 1/[1-c(1-t)] and the transfer multiplier becomes c/[1-c(1-t)]. The transfer multiplier is smaller because only a portion (c) of the transfer is spent initially, whereas government spending enters the economy fully in the first round.