Bob is giving very solid advice and the only correct way to learn what temp to fire any brand of porcelain.. BUT... I don't understand why your oven is so far off.
Have your calibrated your oven, and calibrated correctly? Never used a digital oven that fires so far off the "Suggested" firing parameters.
Of all the ovens I've used, I'm usually firing within 15 C on the higher side..
Its been years since I used Creation but I do remember the first couple of crowns I fired were fired slightly low and Creation chips easily on incisal edges and marginal ridges if under-fired.
You want your opaque to have a sheen no matter what temp it takes and bisbakes should be just shy of being glazed.
The dentin-enamel line your seeing is an easy fix. Creation has a lot of chroma and will have an opaque look if you don't blend enamel down the facial/buccal. Either wipe back with your brush or instrument at an slight angle rather than spooning out to allow for enamel.. That is what you did to see that line ! spooning
Take a good look at the Classical shade guide. Classical /Vita shades have blended enamel going past the incisal 1/3.
As far as your shades being off, that's near impossible with Creation unless you under firing opaques or having grey metal show through/ opaque to thin. Firing dentin at the correct temp is also key to achieving better shades.
Creation has a tendency to appear dark, to much chroma, if you don't blend down the facial, aprox half way or more.
fyi I would suggest going to Chicago for Jensen's convention.. That one weekend will put you years ahead in techniques, and a small price to pay for all that can be learned.
