Hornby make curved track in four radii. You are interested in the smallest.
2nd radius - 438 mm = 17.24 inches (to track centre). This will have the edge of the track very close to the edge of your board. (38/2 = 18)
1st radius - 371 mm = 14.60 inch. This will leave a wider margin at the edges for greater safety from falling and room for scenery.
In either case you want the longest section of track to reduce the number of joins, that is eight 45 deg pieces.
2nd radius curve, 45 deg - catalogue number R607
1st radius curve, 45 deg - catalogue number R605
Note these smaller radii are suitable only for short wheel base stock.