When it’s impromptu there are no rules. Make your intentions very clear. Either speak to your partner or make super emphatic gestures. Reaching to side with your hand and waving the other guy forward will generally be understood. If you wave forward with your left hand while edging over to the right most will get it. But maybe not, avoid assumptions.
In olden days amongst racers pulls were short or shorter. Seconds, not minutes. Ten seconds was not too short and a thirty second pull was a long pull. Currently there seem to be no rules or standards at all.
Try to ride steady. Try to be predictable.
A length back is as close as I want to be with a pickup group. There is more draft when closer but there is plenty at a bike length. Sounds to me like you have the general idea.