Before doing any more drilling/adjusting/moving etc I'd recommend you do a trial run erecting the awning using the pads that came with the Walker awning. Due to the apex being pushed up thereby affording more headroom and the associated left and right side apex poles there's a limit to how far you can extend the poles before you simply run out of awning material. Any placing of fixed brackets screwed to the rail that are outside of this maximum length are simply a waste of time and will serve no purpose. Far better in my opinion to do a dry run with the pads provided first which will enable you to far better judge where the fixed metal brackets will need to go.
As an aside, we've only ever used the supplied pads which have served us very well.

I use the pads on the Dorema awning and they work very well,
I expect the Walker comes with the really good Isabella pads,
so if you don,t like the way the awning sits with your eyed fixings
you can always fall back on the pads. Get out and try it Pete.

Pete, I forgot about the pads, think they're still in the van somewhere as I found them really awkward and so reverted to the brackets. I really don't think you'll have a problem with your marker light as the padded filler strip is soft and will either bend round it or over it. The flaps on either side of it create the close fit to the van when pegged down.
If you need the pads I can look them out and post to you though.

If Pete doesn't want them .....

We haven't even got our new Unico Verona out of the box to play with it yet, because we are too scared to drill our brand new (almost) Eriba to put the brackets on! As it's going to AL the week after next for its first service, I have asked them to do the brackets. Will they know where to put them? or will they have to have the awning out to measure the relevant distance? I guess before we return home from there, we'd better try putting it up to make sure it's all okay - I don't want to be putting it up for the first time in Spain and find the brackets in the wrong place or we have some crucial bit missing! Last year I was new to caravaning and Eribas, this year I will be new to awnings. From what I have observed, erecting them seems to be a 'man thing', so maybe I can just leave the old man to it!

The curved rear 'rafter' of the Unico awnings has quite a lot of available adjustment so I wouldn't worry about the fitment too much Leslie.
I think the brackets can go as far apart as possible without going round the bend!
TonyP