From my personal experience it depends.
For the saddles shoes an 8 in the 2030 last works perfect.
However, for boots I go with an 8.5 and the fit is spot on.
For reference, I do pretty much the same thing with Alden shoes. 8D in Alden barrie lasted shoes and 8.5D in Alden barrie lasted boots. I think this is because with my barrie lasted shoes I wear thin dress socks, whereas with the boots I wear medium weight socks (usually anonymous ism socks). For reference, I measure a true size 9D on the brannock.
Hope that helps.
Thanks, can't decide if I should go 10 or 10.5UK
Normally 10.5 or 10E Barrie