Sir,
As per LTC rules,
1. you will be reimbursed by fare of AC 2 tier superfast charges for the shortest distance.
2. If you go for premier trains like Raj, Duronto, Shatabdi etc. You have to bare fare difference amount.
3....
more...

Your source for return journey and destination must be matching(You cannot avail LTC for circular route).
In your case(Mumbai to Shimla) There is no direct connectivity, hence you can go for break journey @kalka. Since there is direct connectivity to Kalka from Mumbai, you will be reimbursed for AC 2 tier fare for mumbai to Kalka and toy train fare/bus fare from kalka to shimla.
You will get reimbursed with full amount for following plan:
BDTS to KLK: Paschim slip
Kalka to Shimla: Haryana Roadways bus
Return journey:
Shimla to kalka: Himachal Road Transport Corporation bus
KLK to BDTS: Paschim slip
4. The reason for suggesting bus is pretty simple. As per LTC rules, the date at which Paschim slip reaches Kalka should match with departure date for toy train. Since there is no toy train available after arrival of Paschim slip, I suggested bus.
5. There is rule for traveling in bus: You have to travel in bus belonging to state road transport of the the source. Hence I suggested Haryana Roadways bus from kalka and Himachal Road Transport Corporation bus from shimla.
If you really want to travel in toy train, I suggest following for you:
BDTS to klk: Telescopic ticket for Paschim Exp/Paschim slip till New Delhi + Kalka mail till Klk from DLI
klk to shimla: 52451 Shivalik Deluxe Exp
Return journey:
Shimla to Klk: Shivalik Deluxe Exp/Himalayan queen
klk to BDTS: Telescopic ticket for kalka mail till DLI + Paschim Exp/Paschim slip till BDTS.
You can take other train apart from Paschim Exp/Paschim slip as per your convenience and buffer time.
The LTC rules stated above are a bit old. I think some rules got changed/removed(rule 1 and 3 are in effect for sure). So I suggest you to contact clerical staff at your workplace for latest LTC rules, and plan accordingly.