I think the rule of thumb varies with several factors: the number of windings in your coils, the transformer, wire thickness, primary spark gap settings, etc. But I read somewhere that for a medium sized unit the rule of thumb is something like 8.5 Kv per inch, this sounds possible for yours, 8.5 x 12 would put you at about 102,000 volts, a little low for a tesla coil, but possible I think.
There are several internet sites that deal with calculating this voltage based on the various factors directly associated with your Tesla Coil.
We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling We Value Civility
• Positive and compassionate attitudes
• Patience while debating We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving