DNA extracted from FFPE samples tends to be highly fragmented due to the effects of formalin cross-linking. Your best bet is to just try the PCR and see if it works. There are PCR based solutions to determining the degree of fragmentation, by comparing the Ct of known amplicon sizes to your unknown sample.

DNA was extracted from 3 to 4 (5-μm thick) sequential sections for FFPE specimens. Wax was removed from the specimens by adding 300 μL mineral oil to a 1.5-mL micro centrifuge tube containing 3–4 sections of paraffin-embedded tissue, and incubating at 90 °C for 20 min to dissolve the wax. DNA was then extracted with a commercially available kit, DNeasy Blood & Tissue Kit (Qiagen Ltd., West Sussex, UK), using the manufacturer’s protocols, without removing the mineral oil used for de-paraffinization, and eluted using 50 μL of elution buffer. The quality of extracted DNA was examined by agarose gel electrophoresis and ethidium bromide staining, and the concentrations were evaluated by OD 260 (NanoDrop 1000 Spectrophotometers, Thermo Scientific Inc., Waltham, MA).

Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2764035/

The kit that you used is valuable. May be you to investigate the delay of fixation and the fixative used. Besides, you need to perform a PCR to assess the quality of the DNA