Same engine as mine, then. If I drive carefully, I get over 45 MPG (nearly 10 miles/litre). If I drive quickly , 85MPH on the motorway, I get about 39 1/2 MPG ( 8.75 miles/litre).
So, say your diesel costs £1.38/litre, your £20 gets about 14 1/2 litres = 3.22 gallons so I'd expect you to go between 127 and 145 miles.
I don't know how you drive, though! If you're carrying loads around on ordinary roads, or about town, or you drive like a loony or on very soft tyres, you could easily drop to that figure.
FWIW the handbook says:
2.0 16v 110 CV combi, EC directive 93/116:
litres/100km: Urban 8.7 extra-urban 5.6 combined 6.7
MPG Urban 32.5 extra-urban 50.5 combined 42
The 8v JTD uses a little more in the EU and C figures, the 1.9D uses a fair bit more