Welcome to the forum!
You need to divide the tire height addition in half then subtract that from the 1.4" lower to make 1.05" inches lower.
Additionally, springs can/may vary a few mm in drop. A 235/45/17 is 2.8% larger diameter than stock. Generally, you do not want to go more than 3% larger diameter. Hancook makes 10+ tires that start with "Ventus"; which one have you chosen?
My $0.02 - Get Eibach prokit springs and 225/45/17's. IMO the tires will work better on the stock wheels and weigh 3lbs less than the 235/45's. TRD Springs are rebranded Eibach prokit's, which have been proven to work well with the stock shocks. Sportline's will decrease the life of you stock shock and have a rougher ride. This is my opinion. Ignore it if you want.
DIY for spring install.
http://www.ft86club.com/forums/showthread.php?t=10842