Sizing Resistors for Voltage Divider Circuit Feeding Solid-State Relay
Delete R2. The relay can fill that role. 

Let's say that you want to give the relay 5 V. Then you need R1 to drop 43 V out of the total 48 V.

At 5 V the relay will draw about 10 mA according to the graph. So in order to get R1 to drop 43 V at 10 mA, use Ohm's law: 

In this example, R1 needs to be 43 V / 10 mA = 4.3 kΩ.

Also note that R1 in this example will dissipate 43 V * 10 mA = 0.43 W. So you need at least a half watt resistor for this, or combine multiple quarter watt resistors in series (or parallel) to achieve the desired total resistance.