I would not go for a 300 gr head as the rod is really soft, acts as an overcooked spaghetti when trying to throw that 8 wt DT line. I think a 300 gr + weight of the tip + clouser would be too much, really.
Esa, I like your idea of cutting my DT line (in half?) so it would make two heads. Regarding your way of tying it to a mono running line, would you really tie the thick end of the DT taper to the running line (that would make it more of a scandi setup no?)? I thought I'd get more distance tying it the other way around (ie: thin end to running line)? Then is there any rule or formula to get a sink tip that would be right for that custom cut head (or would you just go with a poly leader since the thick part of the head would be attached to the running line)?
I am sorry I missed this post!
If your DT8 is standard its first 30ft weights 210gr and following level line about 7.5gr/ft but because fly lines can have big tolerances you should weight it to be sure. I cast about 225gr shooting heads using #7 single hand rods and about 260gr heads using #8 SH rods which are rated about right. Every fly angler should have a scale because knowing what line weight we cast is good to know but also flyes weight at least for me after boxes get messed up after a while.
Front taper is important for Spey casting but it is almost as important for overhead casting because it slows down line loop run but leader and even fly calms down line loop and line tip does not overshoot too much. It is common that when overhead casting we hear interesting "bang-sound" it is a sign that fly is lost and leader tip speed exceeds speed of sound when false casting and other way casting stroke is began before line hasnot straightened. If flyes are lost and the reason is not the wegetation it can be because casting is too rushed and leader breaks when fly first accelerates and then line tries to jerk it back but can't
I like about 10ft front taper for overhead casting and good DT lines have that long tapers both ends and some have even 15ft tapers but there are DTs which have only 5ft tapers and line tip diameter does not get small either. When fly has lots of wind resistance the leader could be shortened and also shorter front taper line can be better and these improve whe casting to strong head wind.
Overhead shooting head does not need a rear taper but thick line end which is looped can be nasty inside rod when double hauling but most current rods have big enough guides and getting line head out should not need too powerful hauling.
It seems like lines which have longer front tapers and which rears are heavier but not as much heavier than Spey lines are do increase casting distance and it might be because then line loop air drag which is relatively higher on fly leg slows down the line loop run speed and then flight time increase. But this is for distance casting where no fly is used but just a small piece of yarn.
There are no exact formulas how to match tips to bellys but the line loop improves when the front end of a belly is very close the tip rear end weight. You can make other end of your DT a shorter shooting head which you put thin end towards the shooting line and put tip to blunt end. If the tip is tapered as well the head casts smoothly and you can cast it easier some of it inside rod and it still shoots very well because rear is already thin but you might find it easier to cast using also longer overhang which has positive effect to casting distance.