Gary beat me to it!! Or something very like it.
I was wanting to post something like this since I saw a couple of bashas at both the BCUK meet up and the WG that sagged.
This is how I have my basha set up. I have a long line run through all the loops on the central ridge, including the loops on each end, even though they seem turned the wrong way. That main line is tied with an Evenk (sic) and a tensioning knot. The four guylines from the corners are either tied to logs, trees, or pegs. I like the knot that is shown in Ray's Bushcraft book, regular knot used to tension tent guy lines.
Finally, and this is probably where the problem is, the ridge of the basha is tensioned. I have short lengths of paracord, 6-8", tied to the ridge end loops. These lengths are attached to the main ridge support line with timber hitches. The timber hitches can be moved up and down the line with fingers. By sliding the hitches up towards the support trees the basha ridge gets tensioned. In fact you can put enough tension in that the other loops along the ridge don't rest on the ridge support line anymore.
This is very like the arrangement used on the Hennessy to tension their rainfly, only they use a better knot than a simple timber hitch, which is probably what Gary meant :roll:
If anyone has a better description, post it up and I will remove my rather bumbling explanation above :rolmao: