Read it as many times as you need to, but if you're constantly having to go back on every stim, I'd say you're probably not reading closely enough the first time through it. You shouldn't really be racing the clock in LR either. They roughly are put in order of difficulty, so rushing through every question to get through it all will probably lead you to a worse score than taking your time on the easy ones towards the beginning of the section and just guessing on the tough ones at the end.
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You read it as many times as you can until you can locate the premises and the conclusion and understand how the argument arrives at the conclusion.
Read it as many times as you need to, but if you're constantly having to go back on every stim, I'd say you're probably not reading closely enough the first time through it. You shouldn't really be racing the clock in LR either. They roughly are put in order of difficulty, so rushing through every question to get through it all will probably lead you to a worse score than taking your time on the easy ones towards the beginning of the section and just guessing on the tough ones at the end.