The following norms are mentioned in many places now a days as the way to determine one's Ishta:
The deities are worshipped based on the planets in the 12th from kaarakaamsha. Here are >the presiding deities for each planet who will be your Ishta Devata as per your horoscope.
Sun~ Vishnu/ Rama/ Shiva
Moon ~ Krishna/Parvati
Mars ~ Hanuman/ Subramanya/ Sri Narasimha
Mercury ~ Vishnu
Jupiter ~ Vishnu/ Sri Vamana/ Dattatreya/ Sai Baba
Venus ~ Mahalakshmi/ Parashurama
Saturn ~ ShaniDev/ Hanuman/ Kurma
Rahu ~ Durga/ Varaha
Ketu ~ Ganesh/ Matsya
Ascendent ~ Kalki
But the fact is that There is no scriptural basis of this view.
Sri Sai Baba can not be Ishtadevata as per scriptures. Only five forms ie Ganesha, Surya, Vishnu, Shiva and Shakti and their different scripture-approved incarnations are accepted for the Tantrik diksha.
By the way,
anyone can worship one's Guru as God, but can not construct a separate mantra for him or her. Mantras must be mentioned in the scriptures alongwith specifications like Rishi, Chhanda, Devata etc (Reference: Omkarnath Rachanavali, Vol.1, page 293).
Many organisations do have such basisless practices though.
A general guru (who is not a siddha) should judge
Nakshatrachakra, Raashi-Chakra, Rini-Dhani Chakra, Kulaakula-Chakra, Aktaha-chakra, Akamada-Chakra etc.Varahi -Tantra says, Tara-Chakra is required for Vishnu-mantra, Kostha-Chakra for Shiva-Mantra, Rashi-Chakra for Tripura-Mantra, Akamada-Chakra for Gopala/Rama mantra, Hara-chakra in Ganesha-mantra, Kostha-Chakra in Varaha-mantra and Kulakula-Chakra in Mahalakshmi mantra (Ref: Bharitya Saktisadhana O Sakta-Sahitya,Upendrakumar Das, RMIC,Vol.2, page-716).
A siddha-Guru needs no such calculations or anusthanas (ibid, page 753)
According to Diksha-Saara(Amita, republished by Mahamilan Math, Kolkata),
In Vaidika-mantra,Prassada-beeja(HOUM),Pranava, Mala-mantra(a mantra containing more than twenty letters, a mantra received in a dream, Ashtakshara Mantra (OM NAMO NARAYANAYA), Panchakshara-mantra(Namah Shivay) and Mahavidya -mantras(of Kali-Tara etc), no vichara of the above types is required.(page 200).
According to Mantrayoga-Samhita,
Aakashasya adhipo Vishnur agneshchaapi Maheswari/Vayor Agnih kshiter Isho Jivanasya Ganaadipah
meaning : The Adhipati of Aakhasha-tattwa is Vishnu, of Agni-tattwa is Maheswari, of Vayu-tattwa is Surya(Agni), of khsiti-tattwa is Shiva and of Jala-tatwa is Ganeha. The Isha-devata should be chosen by the tattwa which dominates in the body of the disciple (Omkarnath Rachanaavali, Mahamilan Math, Vol 7, page 110-111).
The above can not be determined by an ordinary Guru. In that case
The mantra of the Kula-Devata is advised. If one has special love for some form of God, the mantra of that form can be taken.(ibid : given above)