Resistor in series needed with LED string?
This range and your heatsink 'C/W controls the k= dV/dT for NTC coefficient on LEDs Vf and T ['C] so choose a low side R around 0.1 to 0.5 Ohm depending on heat sink k to avoid thermal runaway. Expect Vf to drop no more than 0.5V /24V at constant current from temp rise due to NTC effects.

A power change without a series R for a 0.2V change in V+ is ΔP=ΔV²/Rs where ΔV =(3.4{=Vf.nom}-2.8{=Vt})/If= 0.6V/0.3A = 2 Ohms
 (24² -23.8²/ (7 * 2 ohms) = 0.71W rise in 7 LED stringfrom 1W to 1.1W using 24V. If LED heatsink is 50'C/W this raises junction 5'C and reduces Vf ~ -4mV/deg C or -20mV so this assumption is thermally stable.

I suggest you use a 0.1V current sense shunt on low side or 0.1V/0.3A= 1/3 Ω and adjust ΔV=0.1V for 300 mA and adjust while hot. You can now measure your thermal resistance by the change in ΔV from the datasheet thermal coefficient ( -3.4 to -4 mv/'C  to ? ) as the string reaches steady state above ambient temp. A good design is case temp below 45'C meaning never too hot to burn finger. But using this method you can measure actual voltage change in Vf. This Vf is the actual change in threshold "knee" voltage Vt not the Rs bulk resistance.

0.3A* 0.1V = 30 mW so you can afford to use 8 LEDs at 3.4V = 27.2V with a 0.5W drop current sense at 0.35A from 29.5V max or R=Pd/I²= 0.5W/0.35²= 4 Ohms 1W with 0.35A*4= 1.4V drop results in 27.2V nom + 1.4= 28.6V  

So if your LED tolerance is on the high side , you have to bypass 1 LED down to 7 and still regulate V+ for current.

I have an empirical formula to determine the heatsink required for CV control to prevent any string from thermal runaway due to NTC Vt effects from temp rise. ( but not included here)  But a long string is less prone to thermal runaway since internal bulk resistance stabilizes the Vf voltage as mainly the threshold internal Vt voltage is affected by temp, not the bulk electrode resitance determined by LED power rating and size..