My tank (not a Patterson) also specifies 290ml, but I usually round it up to 300ml to make mixing the right quantities of chemical that bit easier to calculate.

You don't specify which developer you are using (I only have experience with D-76/ID-11 and DD-X), but assuming you're mixing to the "standard" recommended dilution, don't worry about there not being "enough" developer in that one shot; there is more than enough for one roll - in fact, I frequently develop an extra roll in the same shot with no problems.