The calculation of premium owing at October 1, 2011 doesn't depend on the rate that premium is "used up" in the second half of the year, since the OP will not have "used up" any of that premium yet. The calculation of premium owing at October 1, 2011 is pretty straightforward (assuming the $140 fee is a payment against the policy):
Premium Owing @ Oct 1, 2011:
= (Used Up @ Oct1) - [Paid @ Oct1]
= (0.80 * $4608 ) - [($384 * 7) + $130 + $140] -- b/c he would have paid first and last up front
= $3686.40 - [$2688 + $130 + $140]
= $728.40
Now, it's a little more tricky (and interesting!) to calculate the *actual* new premium being charged to the OP in the second half of the year. The OP's level payments of $130/mo for the remaining five months of the policy do not reflect the actual premium charged, since the payment of $130/mo is also amortizing the balance owing from the first half of the year. It is my guess that the OP's new monthly premium is $90/mo (i.e. when he renews on April 1, 2012, it should drop to about $90/mo).
In order to calculate this new monthly premium (call it X), I backed into it with some Grade 9 Algebra knowing that the policy will be 100% paid by the end of the policy term and also assuming that the $140 adjustment was a payment against the policy. In order for the numbers work out, I also had to assume that State Farm is giving the OP the 25yo rate starting September 1, 2011 (which is quite possible):
Total Paid Premiums @ March 31, 2011
= ($384 * 7) + ($130 * 5) + $140 -- b/c he would have paid first and last up front
= $2688 + $650 + $140
= $3478
Total "Used Up" Premiums @ March 31, 2011
= ($4608 * 0.68 ) + ($X * 0.32) -- assuming that he was given the 25yo rate beginning Sept 1
= $3133.44 + $0.32X
A little bit of Algebra to determine X:
Balance Owing @ March 31, 2011 = $0 (this is a fair assumption)
(Used Up @ Mar31) = (Paid @ Mar31)
$3133.44 + $0.32X = $3478
$0.32X = $3478 - $3133.44
$0.32X = $344.56
X = $(344.56 / 0.32)
X = $1076.75
Thus, the OP's premium when he renews on April 1, 2011 should be about $1077/yr (or $90/mo). Of course, this will go up or down depending on when State Farm started giving the OP the 25yo rate. Now isn't this fun?
OP, you should contact your agent to ask them what your new annualized premium is, just to verify. In other words, if you were to purchase a second identical bike, what would the annual premium be?
I hate State Farm's premium earning method because it's impossible for a layman to understand.