in attempt to keep the peace as i have no problem saying my experience on this matter. I am Workington (doing the challenge) got promotion to Conference after 3 seasons, finished 13th, 18th, 17th and then 11th gradually building up a very good team in the process, at this point my board turned pro during the close season after i finished 11th (maybe a top half finish helps/ ie the board sees that your ready to challenge for promotion (i had also spent much of the season flirting with the playoffs)).
However dont expect the end of season slump too not happen, i was top the whole of the following season but 1 win and 3 draws in 10 games saw me end up 5th though thankfully i scraped up through the playoffs.
In closing, try and progress up the table, once you look like you are going to challenge for promotion / the playoffs they are more likely to turnyou pro, but no guarentees about season-long stamina comes with it