Andrew,
First, I don't think you can go below a full decimal point in Marketo (1%). You'll need to manually create a static list then manually add names (if you're only doing 50, I'd add the second and fifth entries per letter of the alphabet)
Second, let me suggest that an A/B test divided among 50 users is very, very small and the results are not likely to yield much relevant value.
Prior to getting Marketo, we used to subject line testing using groups of between 100 and 250, think that would for for us (we also did not have much of a choice due to a limited data pool).
With Marketo we've learned that to get a decent staistical indicator we need to come in at around 2000 messages per option tested. Based on those results we test with groups of 5-7k more, which then tends to give us some staistical relevance. Hope this helps.