Try XC Doncaster to Birmingham and LM to Watford from there (out of curiousity) - KX to Watford (Met) by tube is not reccomended on time grounds (45+Mins)
I'm not sure where Watford (Met) has come from! The OP did say "Watford" in the title - I will now correct this to Watford Jn to avoid confusion, however the detail did make it clear that the journey is to Watford Jn and the advise has been given accordingly.
I would agree that King's Cross to Watford (Met) would take over 45 minutes, and would certainly not be recommended to get to the centre of Watford, I assume the OP is going somewhere near Watford Jn anyway as that station was identified, but if going to a suburb near the Met station, then the advice would be different.
The Oyster PAYG fares on this route are a bit odd, as customers are penalised for some journeys if going from King's Cross, rather than walking to Euston, but not always. If walking to Euston, there is no morning peak from Euston to Watford High St/Jn.
King's Cross - Harrow & Wealdstone £4.10 M-F 0630-0930 & 1600-1900. £2.70 other times
Euston - Harrow & Wealdstone £3.80 M-F 1600-1900 £2.70 other times
Harrow & Wealdstone - Watford High Street £2.50 M-F 1600-1900 £1.40 other times
Harrow & Wealdstone - Watford Jn £2.50 M-F 1600-1900 £1.40 other times
(It can be cheaper to "split" at [stn]HRW[/stn], however I am unsure how this could actually be achieved, I know there are readers on the footbridge but if they are continuation exit, then I am unsure how you'd split it into 2 journeys)
King's Cross - Watford High St £6.00 M-F 0630-0930 & 1600-1900. £3.50 other times
King's Cross - Watford Jn £7.70 M-F 0630-0930 & 1600-1900. £5.00 other times
Euston - Watford High St £5.50 M-F 1600-1900. £3.50 other times
Euston - Watford Jn £6.90 M-F 1600-1900. £4.10 other times
And for comparison:
King's Cross or Euston - Watford (Met) £5.00 M-F 0630-0930 & 1600-1900. £3.50 other times.
Oyster pricing is incredibly complex, but the good thing about Oyster is that you don't have to figure out the price and buy the "correct" ticket, it works it out for you. The bad thing is, it may charge more than you think if you have not done your research to find out what a journey will cost.