"A mixture of Dowanol DPM and Dowanol TPM will change the evaporation rate". Correct me if I'm wrong but by changing evaporation rate, the diffusion will also slow down?A mixture of Dowanol DPM and Dowanol TPM will change the evaporation rate. You can also include IsoPar M in your mix of solvents. I cannot give you an exact formula of the solvents because it will differ for each fragrance. You will need to test for yourself. If you have the time, and the patience, and the equipment, why not measure the evaporation rate of each of the solvents to see for yourself which is the quickest to evaporate. You need to fill your container with the test material, add reeds and weigh for a number of days so that you can follow weight loss over time.
This may sound laborious, but I assure you this is how it is done. Before we presented any reed diffuser fragrance to a client we would have tested its weight loss over time. Usually we were expected to create fragrances that lasted for a specified length of time, measuring weight loss over time was the only way to check this.