status1:
I do NOT use the TOS Price Slice PnL. But was curious, so poked around a bit. If you click on the P/L Open value, you should see something similar to:
My guess is the "Sim Price" you see here accounts for the difference.
I would GUESS, that during trading hours, the price you observe for MID/MARK may more closely match what TOS would show here as the "Sim Price" as there would be no reason for them to incorporate IV (or Time) in computing price! (This assumes, they don't play games with the Mid/Mark). Note that if you (as a test only), alter the Date, this value changes, providing some hint to what they may be doing behind the scenes!
--
As for that RTD formula: Since I only access BID/ASK prices, and integer values, I do not use it. (I do my own Greeks) --
If you use TOS greeks, you may wish to tweak your RTD references similar to this to get better precision:
=RTD("tos.rtd",,<nugget>,"100*<opra>")/100
Where <nugget> is your desired item, such as DELTA, IMPL_VOL, etc.
Note: TOS has been known to alter their handling of IV without disclosing, so this may be a moving target (one reason I stay away from it).