Savings in converting from steam to hot water is a result of lowering the temperature of the heating medium from 215F to about 180F with reduced losses from the boiler and piping. Also, the temperature of unocupied zones can be reduced while the occupied areas are heated. For boilers of equal efficiency, steam to water, 25% savings are possible. Old boilers were about 45% AFUE efficient, so more savings can be assumed.
Baseboard output can be accepted as 550 btuh per linear foot, so the heat loss of the room divided by 550 will help achieve a correct baseboard length. This information can be found step-by-step at the free lessons on http://www.hydronic.net
where you can also do a room-by-room heat estimate.