You need to look at the installation
specitications for the thruster you have purchased. They will generally have both a minimum depth
below the waterline requirement and a minimum tube length requirement. The minimum tube length will dictate how far down and forward you can go before the hull
gets too narrow. You will want to install the thruster as far forward as possible while still being within these limits. You will also need to take into account what bulkheads and furniture are inside your mounting area that might get in the way. For example, you might be within the limits far forward but the anchor locker
bulkhead might be in the way. In that case you will need to come back a bit to avoid it.