Looking at the literature, we can see that in Magi of Hermes, p45, we have:
Move Gigantic Wicker Basket Which is a ReHe movement item. Now, while it's effectively enchanting an elevator, it can provide a literature-supported appropriate baseline for our item.
ReHe 48 Pen +0, unlimited uses R: Touch, D: Momentary, T: Circle When
the right mental command is made, the enchanted ring of stones moves
the gigantic wicker basket carriage 50 pac- es in some direction. The
destination is set when the enchantment is put down. (Base 15, +1
Touch, +3 Size, +10 lvls Unlimited uses +3 lvls Linked Trigger)
We can see base 15, which matches no CrAu, but instead ReCo as a transport spell. We note that the spell moves instead of teleports, providing the necessary simulacrum of locomotion.
Therefore, is is reasonable to assume that a broomstick with this ReCo equivalent ReHe can provide impetus to its rider.
My player and I settled on:
Base 20, +1 concentration, +5l device maintains concentration, +10 unlimited
This provides an impulse or impetus of 500 paces per six seconds, (translation: wheeee!) and is controlled by pointing it in the right direction. Due to a correspondence with tradition, the broom is "started" by lighting the candle (and the candle will gradually fade at sunrise and sunset, suggesting that the magi should probably get out of the sky.) It's speed can be controlled by dimming or trimming the candle or its flame. All of this, of course, is purely thematic and is reflected in the item only such as that the device maintains concentration and effectively "recasts" the spell every time the change is made. It would be bad for this broom made of apple wood and wicker (iBroom) to run into ("You have reached your speed modifications per day limit.") problems with adjusting speed.
At 35, this item will be about half as fast as wings of the soaring wind. At 40, it's rather faster, just as wings would be at +1 mag.