The simulator allows for fractional ranges. If your starting requirements are 1-QB, 2-RB, 2-WR, 1-TE, 1-WR/RB, 1-K 1-DST, we generally suggest entering 1-QB, 2.5-RB, 2.5-WR, 1-TE, etc...
The roster numbers you enter affect the value of each position relative to each other, so if you are a person who favors a WR over RB in your flex - or if your league generally favors or over-drafts WRs - you might also try variants of this, such as: 1-QB, 2.3-RB, 2.7-WR, 1-TE, etc...
These subtle changes affect how each position is valued relative to other positions. In this matter, it's more art than math and there is no strictly "correct" answer. Try out different possibilities and see how they fit into your personal tastes or unique league's needs. The most important thing here is that the numbers add up correctly to the total number of roster slots.
As another example, if your starting roster requirements were 1-QB, 2-RB, 2-WR, 1-TE, 1-WR/TE, 1-WR/RB, K, DEF, you might try: 1-QB, 2.5-RB, 3-WR, 1.5-TE etc... (If you prefer WRs over TEs in the WR/TE flex, you might try entering 3.2-WR and 1.3 TE, or similar possibilities)