What supports the period after ὃ γέγονεν is that ζωὴ is feminine. What supports the period before ὃ γέγονεν is the poetic/phrase structure.
The decisive factor is putting the period before ὃ γέγονεν seems to conflict with the context. It seems to say all that was created was life. It assigns both γέγονεν and ἦν to ζωὴ without reason or explanation. Whaile it's not impossible for John to have neuter pronouns with the feminine ζωὴ as the antecedent, it is unlikely when doing so would cause such ambiguity. Putting the period after ὃ γέγονεν implies that John meant life to be an uncreated attribute of God (Matt. 16:16), thus life as an attribute of the Word. Therefore, the period after ὃ γέγονεν makes the most sense.
Poetic Sentence Structure
Look at how NA28 punctuation fits the structure of the prolegomena well (shown at the end). What seems to fit the context best is the ἐν prepositional clause being instrumental, and a translation would be something like "Life which was made by him was," or "there was life which was made by him." The problem with a translation, "That which was made by him was life," seems to say life was all that he made, which contradicts the previous sentence.
However, there is a problem with translating ζωὴ to mean all created life. It gives ζωὴ a different meaning than ἡ ζωὴ in the next sentence. Thus, better might be, "That which came by him was life, and that life was the light of humanity."
Ἐν ἀρχῇ ἦν ὁ λόγος,
καὶ ὁ λόγος ἦν πρὸς τὸν θεόν,
καὶ θεὸς ἦν ὁ λόγος.
οὗτος ἦν ἐν ἀρχῇ πρὸς τὸν θεόν.
πάντα διʼ αὐτοῦ ἐγένετο,
καὶ χωρὶς αὐτοῦ ἐγένετο ⸂οὐδὲ ἕν.
ὃ γέγονεν ἐν αὐτῷ ζωὴ ἦν,
καὶ ἡ ζωὴ ἦν τὸ φῶς τῶν ἀνθρώπων·
καὶ τὸ φῶς ἐν τῇ σκοτίᾳ φαίνει,
καὶ ἡ σκοτία αὐτὸ οὐ κατέλαβεν.
ἀπεσταλμένος παρὰ θεοῦ,
ὄνομα αὐτῷ Ἰωάννης·
οὗτος ἦλθεν εἰς μαρτυρίαν
ἵνα μαρτυρήσῃ περὶ τοῦ φωτός,
ἵνα πάντες πιστεύσωσιν διʼ αὐτοῦ.
οὐκ ἦν ἐκεῖνος τὸ φῶς,
ἀλλʼ ἵνα μαρτυρήσῃ περὶ τοῦ φωτός.
Ἦν τὸ φῶς τὸ ἀληθινόν,
ὃ φωτίζει πάντα ἄνθρωπον,
ἐρχόμενον εἰς τὸν κόσμον.
ἐν τῷ κόσμῳ ἦν,
καὶ ὁ κόσμος διʼ αὐτοῦ ἐγένετο,
καὶ ὁ κόσμος αὐτὸν οὐκ ἔγνω.
εἰς τὰ ἴδια ἦλθεν,
καὶ οἱ ἴδιοι αὐτὸν οὐ παρέλαβον.
ὅσοι δὲ ἔλαβον αὐτόν,
ἔδωκεν αὐτοῖς ἐξουσίαν τέκνα θεοῦ γενέσθαι,
τοῖς πιστεύουσιν εἰς τὸ ὄνομα αὐτοῦ,
οἳ οὐκ ἐξ αἱμάτων
οὐδὲ ἐκ θελήματος σαρκὸς
οὐδὲ ἐκ θελήματος ἀνδρὸς
ἀλλʼ ἐκ θεοῦ ἐγεννήθησαν. (John 1:1-18, NA28)
Καὶ ὁ λόγος σὰρξ ἐγένετο
καὶ ἐσκήνωσεν ἐν ἡμῖν,
καὶ ἐθεασάμεθα τὴν δόξαν αὐτοῦ,
δόξαν ὡς μονογενοῦς παρὰ πατρός,
πλήρης χάριτος καὶ ἀληθείας.
(Ἰωάννης μαρτυρεῖ περὶ αὐτοῦ καὶ κέκραγεν λέγων· Οὗτος ἦν ⸂ὃν εἶπον·⸃ Ὁ ὀπίσω μου
ἐρχόμενος ⸆ ἔμπροσθέν μου γέγονεν,* ὅτι πρῶτός μου ἦν.)
ὅτι ἐκ τοῦ πληρώματος αὐτοῦ ἡμεῖς πάντες ἐλάβομεν
καὶ χάριν ἀντὶ χάριτος·
ὅτι ὁ νόμος διὰ Μωϋσέως ἐδόθη,
ἡ ⸀χάρις καὶ ἡ ἀλήθεια διὰ Ἰησοῦ Χριστοῦ ἐγένετο.
θεὸν οὐδεὶς ἑώρακεν πώποτε·
μονογενὴς θεὸς ὁ ὢν εἰς τὸν κόλπον τοῦ πατρὸς