You're right,
Halsebee, it's the outward portion, not the return, which needs to be excessed and it would be easy to be charged more than necessary for this journey by mistake. There are three different routes which are priced for a Taunton - Worthing journey and two different fare types come into the calculation.
Both legs of your journey are intended to be made on days when no time restrictions apply. Therefore relevant fares for the whole journey are:
National Fares Manual NFM 08 CD said:
Taunton - Worthing
Route + Any Permitted
Off-Peak Return (SVR) £88
Super Off-Peak Return (SSR) £73
Route Not London
Off-Peak Return (SVR) £85
Super Off-Peak Return (SSR) £71
Route Salisbury
Off-Peak Return (SVR) £45
So the ticket combination that you want is a £45 SVR route Salisbury, with an excess of the outward leg to a £71 SSR route Not London, which should cost half the difference: £13. This makes a total of £58.
One or two points of potential difficulty. First it has occasionally been reported on this forum that the full difference rather than half the difference has been charged for a change of route excess. This has probably been due to ticket office staff being unfamiliar with excess fare procedures which they use infrequently. Fortunately, this problem appears to have been resolved by a new generation of ticket machines which calculate the amount of the excess fare automatically.
Second, it has on one occasion been asserted here that it is not permitted to excess a SVR to a SSR fare. Others have disagreed and there is nothing in the excess fare procedures to justify the assertion. We are in the fortunate position that one train company's internal excess fare procedures have been made public as a result of a request under the Freedom of Information Act. East Coast's procedures are attached.
If I were making this journey, I would attempt to buy the £45 ticket and the £13 excess in the same transaction at a ticket office before starting the journey. If not offered the correct excess fare, I would buy the £45 ticket only and excess it during the journey. On-train staff tend to be more familiar with excess fare procedures. Condition 13(e) of the National Rail Conditions of Carriage gives you the right to obtain an excess at no extra charge on the train.
National Rail Conditions of Carriage said:
B. VALIDITY OF TICKETS
13. The route you are entitled to take
(e) If you make your journey by a route other than those referred to in (a) and (b) above, you will be liable to pay an excess fare. This excess fare will be the difference between the price paid for the ticket you hold and the price of the lowest priced ticket(s) available for immediate travel that would have entitled you to travel by that route.
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
Don't pay too much! There is no refund due if you choose to use a more expensive ticket over a cheaper route.