Masking lines can be one of the most frustrating things to do.
When I built Zouki-Muras Ta 152 H-1 last year I decided that the defence of the Reich Bands would look much better painted than the decals from the sheet and I spent quite a long time and lots of tape getting it right.
You can see that the red band has already been done. Luckily on this aircraft the bands were defined by panel lines, but it still needs time and care starting with very thin yellow tape to define the band, working upwards in size tomask off a large area to catch overspray. White, Red and Yellow are terrible colours to work with!
The Yellow band was marked out starting with 0.2mm yellow tape with the end of the red being the start for that side. theedge of the band nearest the cockpit is at an angle ( I did work from both the model and the decals to get it right) and you can see the amount of various widths of tape I used
Once the tape was removed.....
The finished result....
Curved surfaces are awkward which is why I start with the very thin yellow tape as it will curve. So long as the edge bordering where the stripe/line will start is flat to the surface then your paint will have a good edge and not bleed (hopefully). You may need to nick the opposite edge to get the tape to lie flat over a curved 3d surface. Always mask AWAY from the line not towards and be prepared to nick and cut edges to get smooth contact rather than ridges and bumps.
I use cocktain sticke and using a knife make a flat sdge for smoothing over tape rather than my finger... especially edged where paint is going to go.
Maybe these will be of use...
http://www.lmg.in.ua/gb/building-berth/13-stapel-dlya-sborki-aviamodelej.html
http://www.lmg.in.ua/gb/building-berth/46-lmg-bb-06-stapel-dlya-sudomodelej.html
http://www.lmg.in.ua/gb/building-berth/19-stapel-dlya-btt.html
Maybe with a bit of thinking outside the box you can combine these to mark out with a pencil ot to help mask.