FYI: For Evocation grapples, the block value is based on shifts of power, not shifts of power + effect.
If you only do 6 shifts (3 for block and 3 for duration), it is a block of value 3 that lasts for your next 3 exchange actions (unless ended prematurely by an escape). But you don't have to maintain it beyond the initial exchange.
A weak block might not be worth the exchange action or the stress you suffered for it, so this is a time to "up the ante" and invoke a few aspects to get the discipline check up there (7 block for 3 exchanges is better and worth the stress).