Italian long distance trains (IC €3 and Frecce €13) need reservations, so probably Rome-Pisa and Florence-Venice. Munich-Düsseldorf and Düsseldorf-Amsterdam have optional reservations but they may be recommended depending on the time of the day (€3 via tickets.oebb.at or free of charge in 1st class).

Reservations Manchester-London are optional and can be booked free of charge.

If reservations are mandatory, then they're mandatory for anyone. But if you buy regular tickets, then the reservations will be included automatically. If you buy a Eurail pass, reservations are only mandatory for trains that require it.

here's the crazy thing. i just checked the reservation cost for duss - paris and its 37 USD with a pass.  
The ticket on Thalys is only 32 euros all up which includes the reservation cost how on earth is a Eurail pass cheaper. Imce i add up all the reservations cost i can see its not really going to be a savings at all
 

From your previous post I gather you're travelling at the end of December. Thalys can't be booked yet for those dates so can't look up the fare yet. Prices in 2nd class for Thalys Düsseldorf-Paris start at €32 but go all the way up to €145. You'd be very lucky if you can find such a cheap ticket for such a busy travel period, even if you book in advance.

Of course, you can find cheap(ish) tickets if you book in advance, but those tickets will often be non-refundable and non-changeable, whereas a pass is flexible. If you want to do a calculation, then it's too early. You could do a rough estimation and check fares the number of weeks from now that you'd be willing to book in advance. But it will still be a very rough estimation because the whole holiday period at the end of December/beginning of January is a very busy travel period.

Yes thats what im doing im checking the prices for every single journey plus ive calculated which journeys require reservation costs which is quite a few of them.
after this i am deciding which journey would be a Eurail journey and im working out this against the 4/5/7 day passes. My spreadh sheet is looking pretty detailed by now and yes i too agree that prices might change but looking ahead we are going to be in the middle of winter. im not sure if prices are cheaper over there then but ive looking currently at bookings that are as far advanced as i can go so at this stage i'm looking in september
At the moment choosing the best options on the  4 day pass its coming out at 1502 USD for  3 ppl for all journeys combined with reservations against 1464 USD doing them separately at the best pricing currently 
I am still working out the 5 and 7 day options