The lowest usable flight level is the transition level of FL40.
How?
1013.2 - 1010 = 3.2 Mb
3.2 * 30 = 96ft
The position of FL30 is 96ft below (pressure is lower) 3000ft AMSL.
Therefore, FL35 is sitting at 3404ft AMSL and is thus 404ft above the transition altitude.
Thus FL35 is the transition level and is the lowest useable flight level.
However, remember that cruising at FL35 will only provide 404ft vertical separation from flights cruising at 3000ft QNH.
Regards,
DFC