Hi Conor,
Welcome to the TI E2E forums!
The internal clock for the ADS1299 is not a parameter that the user can control. The device is trimmed to have a nominal frequency of 2.048MHz (tCLK = 488 ns). The range for tCLK that you see in the Timing Requirements table is the allowable tolerance on this spec and it is not linearly mapped to the range of DVDD.
The same allowable range for tCLK is valid when using an external clock as well.
As you look through the Theory of Operation section of the datasheet, many of the important timing specs are simply expressed as functions of tCLK.
Regards,